In practical terms, to achieve high speed duplex printing of paper sheets using inkjet print heads, it is necessary to use multiple print heads. Because conventional inkjet print heads print “down”, it is necessary to turn a paper sheet between the action of a first print head printing on one surface of the sheet and a second print head printing on the reverse surface of the sheet. Turning a cut sheet over in a printer to enable printing on one side after printing on the other side is a difficult action at even moderate speeds. Moreover maintaining paper registration so that the front and back images are where they are supposed to be, is a challenging requirement.